At present, metal sheets such as electrotinplate, tin free steel and aluminum sheet are widely used for can stock after being coated, at least once, with lacquer. This lacquer coating is disadvantageous from an energy standpoint as significant time is required for curing the lacquer and large volumes of solvent discharged during the lacquer curing process must be burned in another furnace in order to prevent air pollution.
Recently, lamination of thermoplastic resin film on a metal sheet was attempted in order to avoid these problems. For example, the methods shown in Japanese Patent Publication Nos. Sho 61-3676 and Sho 60-47103,, Laid-Open Japanese Patent Application Nos. Sho 61-149341 and Hei 1-249331 are already known.
Japanese Patent Publication No. Sho 60-3676 relates to a process for lamination of a polyester resin film onto a tin plated steel sheet which comprises preliminarily laminating the polyester resin film to a tin plated steel sheet at below the melting temperature of tin, and then reheating the laminate at above the melting temperature of tin in order to completely bond the film to the tin plated steel sheet.
Japanese Patent Publication No. Sho 60-47103 relates to a process for lamination of a crystalline polyester resin film to a metal sheet such as tin free steel and electrotinplate by heating the sheet above the melting point of said polyester resin film and thereafter immediately quenching the laminate.
Laid-Open Japanese Patent Application Nos. Sho 61-149341 and Hei 1-249331 relate to a process for lamination of a polyester resin film precoated with a special adhesive, such as an epoxy resin containing a curing agent to a metal sheet such as tin free steel and electrotinplate which has been heated below or above the melting temperature of said polyester resin film.
The electrotinplate covered with the polyester resin film obtained by these patents above is not suitable for applications wherein excellent corrosion resistance after severe forming is required because the adhesion of the polyester resin film after severe forming is inferior to that in the polyester resin film laminated tin-free steel. Particularly, in the electrotinplate in which the polyester resin film is laminated at above the melting temperature of tin, the adhesion of the polyester resin film after severe forming is noticeably poor, because it is considered that the adhesion between the tin layer and metallic chromium layer, which is present on the tin layer, becomes poor by melting of a tin or an iron-tin alloy layer which forms by heating during the lamination of the polyester resin film is destroyed by severe forming. In the electrotinplate in which the polyester resin film is laminated at below the melting temperature of tin, the laminated polyester resin film is easily peeled off by severe forming, even if said TFS film is formed on a plated tin layer, because the adhesion of the tin layer to the metallic chromium layer formed on the plated tin layer is poor compared with the adhesion of the metallic chromium layer to the steel surface in tin-free steel.
On the other hand, in the polyester resin film laminated tin free steel obtained by these patents described above, the laminated polyester resin film is not peeled off by severe forming, if the polyester resin film having excellent formability is laminated. However, in the process wherein the polyester resin film is laminated on tin-free steel or the polyester resin film laminated tin-free steel is formed, the impurities such as dust and steel powder may be mixed in the interface between the laminated polyester resin film and the surface of the tin free steel or may stick on the surface of the polyester resin film laminated tin free steel. These impurities may act as the starting point for the birth of many cracks in the laminated polyester resin film during severe forming. These cracks deteriorate the corrosion resistance of the polyester resin film laminated tin free steel. For example, when some corrosive drinks such as a carbonated beverage or a sports drink is packed in the deeply drawn can made by the polyester resin film laminated tin free steel and then are stored for about one month at room temperature, perforations may arise from many cracks in the laminated polyester resin film of the formed part in the drawn can, because the TFS film does not prevent the corrosion of base steel exposed by severe forming which occurs electrochemically.
Therefore, the tin free steel covered with the polyester resin film is not suitable for industrial applications where excellent corrosion resistance after severe forming is required. If the polyester resin film laminated tin free steel is used for applications described above, the environment of the processes for laminating the polyester resin film on the tin free steel and forming the polyester resin film laminated tin free steel must be sufficiently controlled in order to prevent the mixture of these impurities into the polyester resin film laminated tin free steel.